Award: OCE-1437054

Coral reefs provide crucial ecosystem services that are becoming increasingly threatened by climate change. Many important questions about coral reef biology remain unresolved and the ?paradox of the coral reef? as it relates to the observation of high productivity in oligotrophic waters is still not fully resolved. It is now well established that the corals harbor many functional groups of bacteria that are associated within the tissues of corals, and the function of these bacteria from an organismal and ecosystem perspective remains largely unknown. One of the reported functions for these symbiotic bacteria is nutrient cycling, and in particular nitrogen, within the coral. The nitrogen cycle in the oceans is mediated principally by bacterial transformations of organic and inorganic sources of nitrogen or by inputs of new nitrogen from terrestrial, or atmospheric sources. In coral reef systems new nitrogen is obtained principally from nitrogen fixation which is then made available to other reef organisms through the release of dissolved organic nitrogen (DON). With the recent discoveries about the presence of nitrogen fixing bacteria in the tissues of corals it begs the question; how important is the contribution of new nitrogen from corals to the surrounding water column and nearby benthic environments? In particular, what are the contributions of nitrogen fixation in corals to the DON content of released mucus? In this project we experimentally assessed how different symbiotic bacterial communities affect the availability of nitrogen, specifically through nitrogen fixation, within the coral and how nitrogen fixation contributes to the DON content of excreted mucus. Using sophisticated genetic sequencing approaches we quantified the taxonomic and functional diversity of these symbioses in corals from the Pacific and Caribbean as well as the taxonomic diversity of the nitrogen fixation gene, nifH, in the tissues of corals. Additionally, we measured the rates of nitrogen fixation in corals from Australia, Hawaii and Curacao and the amount of translocated nitrogen to the different tissues of the coral as well as the contribution of DON released to the environment. The data showed that in all locations corals fixed nitrogen and utilized it to support their metabolism as well as releasing it to the environment for other members of the coral reef to utilize. Additionally, we observed evidence that many corals not only have a core microbiome of symbiotic bacteria but that that microbiome also contained bacteria that consistently fixed nitrogen whether corals were found in the Pacific or the Caribbean, suggesting that the association with nitrogen fixing bacteria precedes the separation of the Caribbean and Pacific oceans. Last Modified: 11/02/2018 Submitted by: Michael P Lesser